Keira turns the air blue

Keira Knightley may be dressed like a lady, but her behaviour was anything but when she swore on the set of her latest film The Duchess.

The Oscar-nominated actress is playing Georgiana, the fifth Duchess of Devonshire and great-great-great-great aunt of Diana, Princess of Wales.

On set in London earlier this week, Knightley cursed and burst into laughter after fluffing her lines during a crowd scene.

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The movie beauty has admitted in the past she hates the early days of filming and feels nervous in front of a crowd.

"I fear everything. New characters, new parts, first days of filming and premieres always terrify the life out of me. Small things terrify me: If I go out, I never know what to say," Knightley said.

"I always feel self-conscious," she added.

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Knightley was set to take an extended break from acting this autumn after filming a string of movies over the past few years, including the Pirates of the Caribbean sequels, Atonement and The Edge of Love, but changed her mind after she was sent the script for The Duchess.

The movie has been adapted from Amanda Foreman's biography of the 18th century aristocrat, who endured her husband, the Duke of Devonshire's affair with her best friend, Lady Elizabeth 'Bess' Foster. Their unhappy marriage inspired Richard Brinsley Sheridan's play School For Scandal.

Ralph Fiennes is playing Knightley's on-screen husband, while Charlotte Rampling is playing Lady Spencer.

The Duchess marks Knightley's sixth time in corsets, after donning the period garb in Pride and Prejudice, the Pirates trilogy and the unreleased Silk.

In 2005, she complained: "I could be stuck in corsets for the next 20 years."